Microcomputers (personal computers) are being required to perform more and more complex data processing tasks while not suffering a degradation in response speed. On the other hand, in order to perform the more sophisticated tasks, additional device drivers, networking programs, host, attachment programs, session managing programs, etc., must be loaded into the available memory space. The amount of space left for the user's applications programs is thus shrinking, forcing undesirable trade-offs to be made among storage, performance, and function.
To remedy this problem, microprocessors have lately become available in which the amount of addressable memory has been greatly expanded. For example, for the Intel Corporation 8088/8086 microprocessor (hereinafter a microprocessor will be referred to for convenience as a "CPU"--central processing unit). the amount of addressable memory is about 1 MB (MegaBytes), while for the newer 80286, CPU, about 16 MB can be addressed.
The 80286, however, employs two different and mutually incompatible addressing modes. The first mode, termed the "real" mode, is exactly the same addressing mode employed in the 8088/8086 CPU, and hence programs written for a machine employing the 8088/8086 CPU, such as the vast array of software written for the IBM PC computer and compatibles, can be run in the real mode since the same BIOS (Basic Input-Output System) can be used directly. In the real mode, however, since the addressing mode is in fact the same as for the 8086/8088, the amount of addressable memory is still limited to about 1 MB.
The second mode, termed the "protected" mode, employs a different memory addressing scheme, and with this scheme can address up to about 16 MB of memory. However, because the addressing mode is indeed different, the earlier BIOS cannot be used successfully, and hence computers which have employed the 80286 CPU have not been able to simultaneously take advantage of the increased amount of available memory in the protected mode and run software written for the 8086/8088 CPU.
FIG. 1 shows a memory map of a typical microcomputer application employing an 80286 CPU and showing an example of how the memory may be organized. Memory addresses in the range of 0 KB (KiloBytes) to 40 KB are taken up by the BIOS (Basic Input-Output System) and OS (Operating System). the most famous examples being PC DOS and MS DOS marketed by Microsoft Corporation. The user is allocated the space from 40 KB to 640 KB in both the real and protected mode. The video buffers occupy 640 KB to 752 KB, and feature and planar ROMs (Read-Only Memories) 752 KB to 1 MB. This is all the memory that can be addressed in the real mode. In the protected mode though 15 MB of additional addressable memory space is available to the user.
To better understand the problem solved by the invention, the two addressing modes will now be described in more detail.
For a CPU using segment and offset registers for addressing physical memory can be addressed directly using 32-bit pointers. As shown in FIG. 2, each 32-bit pointer is composed of a 16-bit offset (bits 0 to 15) and a 16-bit segment (bits 16 to 31). The memory is divided into 64 KB segments, and each of the 16-bit segment values of the pointers corresponds directly to one of these 64 KB segments in memory. That is, pointer segment n, multiplied by 2.sup.4 (equivalently, shifted one place in hexadecimal), directly indicates the address of the first eight-bit byte of data in segment n of the memory, namely, the boundary between segments n-1 and n in physical memory. The offset, on the other hand, indicates a displacement from the boundary between segments n-1 and n.
In the real mode of the 80286 CPU as indicated by the diagram of FIG. 3, to obtain the 20-bit value which directly addresses a given byte location (operand address) in physical memory, the segment value is multiplied by 2.sup.4 and added to the offset value. This 20-bit address is applied directly to the memory as an address.
In the protected mode of the 80286, the BIOS does not use physical memory in the form of segments and offsets. Moreover, the memory is not divided up into 64 KB segments. Instead. "virtual" memory addressing is employed in which the addresses do not correspond directly to distinct locations in physical memory. To allow for more efficient use of the available memory space while still retaining relative ease of addressing, the memory is again divided into segments, but the segments may be of variable lengths. Generation of the actual physical addresses is done internally to the 80286 CPU, out of reach of the user and BIOS.
The protected addressing mode will be explained in more detail with reference to FIG. 4 of the drawings.
As in the case of the 8088/8086 and real mode, the BIOS addresses memory using a 32-bit pointer. In the protected mode, the lower 16 bits (bits 0 to 15) of the pointer are also referred to as an offset. Because its function is different than that of the segment in the 8088/8086 and real mode the upper 16-bit portion of the pointer is termed a "selector". Instead of merely multiplying it by 2.sup.4 and adding it to the offset, the selector is used as a pointer to a segment descriptor contained in a segment descriptor table, which is assembled in a predetermined area of the physical memory. Each segment descriptor contains a 24-bit value, which indicates the base address (lower boundary address) of the corresponding segment in physical memory. To obtain the actual address of a desired operand in physical memory, the 24-bit segment descriptor value retrieved by the selector is added to the offset.
Because the selectors employed in the protected mode thus have a function which is much different than and nonequivalent to that of the segments in the 8088/8086 and real mode, the BIOS designed for the 8088/8086 and real mode, which uses segments in addressing, cannot operate directly in the protected mode, making it impossible to run in the protected mode programs written to use the BIOS developed for the 8088/8086 and real mode. While such programs can of course be run on the 80286 CPU in the real mode, it is a disadvantage not to also be able to run them in the protected mode.